• Transactions of Materials Processing
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• v.29 no.6
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• pp.323-330
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• 2020

The surface region of a sheet metal may have different characteristics from the inner region because the surface region is less restricted than the interior. In addition, the grains on the free surface are less hardened because of surface adsorption of the dislocations, rather than piling up. In the case of bulk or thick sheet metals, this effect is negligible because the fraction of the surface region is much smaller than that of the inner region. However, this surface effect is important in the case of ultra-thin sheet metals. In order to evaluate the surface effect, tensile and bending tests were performed for the SS304 stainless steel with a thickness of 0.39 mm. The bending force predicted using the tensile behavior is higher than the measurement because of the surface effect. To account for the surface effect, the surface layer model was developed by dividing the sheet section into surface and inner layers. The mechanical behaviors of the two regions were calibrated using the tensile and bending properties. The surface layer model reproduced the bending behavior of the ultra-thin sheet metal.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.8
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• pp.2113-2122
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• 1994

The 3-D FEM analysis for simulating the stamping operation of planar anisotropic sheet metals with arbitrarily-shaped tools is introduced. An implicit, incremental, updated Lagrangian formulation with a rigid-viscoplastic constitutive equation is employed. Contact and friction are considered through the mesh-normal, which compatibly describes arbitrary tool surfaces and FEM meshes without depending on the explicit spatial derivatives of tool surfaces. The consistent full set of governing relations, comprising equilibrium equation and mesh-normal geometric constraints, is appropriately linearized. The linear triangular elements are used for depicting the formed sheet, based on membrane approximation. Barlat's non-quadratic anisotropic yield criterion(strain-rate potential) is employed, whose in-plane anisotropic properties are taken into account with anisotropic coefficients and non-quadratic function parameter. The planar anisotropic finite element formulation is tested with the numerical simulations of the stamping of an automotive hood inner panel and the drawing of a hemispherical punch. The in-plane anisotropic effects on the formability of both mild steel and aluminum alloy sheet metals are examined.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.10a
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• pp.369-372
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• 2005

A study on the microstructure, the texture and the formability of the samples after ECAPed and subsequent heat-treated AA 1050 aluminum alloy sheet have been carried out. The specimens after the ECAP showed a very fine grain size, a decrease of <100> // ND. The <110>// ND textures appears in the specimens after the ECAP and subsequent heat-treatment at $400^{\circ}C$ for 1 hour. One of the most important properties in sheet metals is formability. The r-value or plastic strain ratio has was as a parameter that expressed the formability of sheet metals. The change of the plastic Strain ratios after the ECAP and subsequent heat-treatment conditions were investigated and it was found that they were two times higher than those of the initial Al sheets. This could be attributed to the formation above texture components through the ECAP and subsequent heat-treatment of AA 1050 Aluminum alloy sheet.

• Transactions of Materials Processing
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• v.14 no.6 s.78
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• pp.553-558
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• 2005

A study on the microstructure, the texture and the formability of the samples after ECAPed and subsequent heat-treated AA 1050 aluminum alloy sheet have been carried out. The specimens after the ECAP showed a very fine grain size, a decrease of <100> // ND, and an increase of <111> // ND textures. The $\{111\}<112>,\;\{123\}<634>,\;\{110\}<001>,\;\{112\}<111>,\;\{110\}<111>,\;and\;\{013\}<231>$ texture components were increased in the specimens after the ECAP and subsequent heat-treatment at $400^{\circ}C$ for 1 hour. One of the most important properties in sheet metals is formability. The r-value or plastic strain ratio has was as a parameter that expressed the formability of sheet metals. The change of the plastic strain ratios after the ECAP and subsequent heat-treatment conditions were investigated and it was found that they were two times higher than those of the initial Al sheets. This could be attributed to the formation above texture components through the ECAP and subsequent heat-treatment of AA 1050 Aluminum alloy sheet.

• Korean Journal of Metals and Materials
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• v.46 no.6
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• pp.377-381
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• 2008

Prism sheet of LCD BLU which depends on supply from Japan and U.S.A was studied by using Si anisotropic etching and injection molding technologies. First, the prism sheet was patterned on Si wafer through photolithography, and the best conditions of Si etching were determined through etching Si wafer with TMAH to obtain straight optimized zigzag patterns, and a cross pattern to provide light diffusion and concurrent focusing. The etch rate of TMAH was concluded to be constant for $25wt%-70^{\circ}C$ condition. Ni stamp of prism sheet was made by electrodeposition using patterned Si wafer, normal or fast H/C(Heating/Cooling) injections were carried out to fabricate prism sheet. It was known that fast H/C injection could fabricate prism sheet more accurately than normal injection. Zigzag patterns and the cross pattern showed higher transmissivity than the straight patterns because of light diffusion through diagonal direction. The fast H/C injection for zigzag patterns showed lower transmissivity than normal injection because there occurred more light diffusion through precise injection patterns, but the fast H/C injection for straight patterns showed only refraction without diffusion, causing lower transmissivity than normal injection.

• Korean Journal of Metals and Materials
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• v.48 no.2
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• pp.116-121
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• 2010

The Mg-Zn-RE alloy cladded with the thin Al1050 sheet was fabricated by means of a roll bonding process at $280^{\circ}C$.Microstructures and mechanical properties of the clad sheets were investigated. After heat treatment at $230^{\circ}C$ for 30 min, an Mg-rich diffusion layer with about $2{\mu}m$ in thickness was developed at the Mg and Al interface. Tensile tests were carried out in a temperature range up to $300^{\circ}C$. The clad sheet exhibits superior elongation to failure not only at room temperature but also at elevated temperatures compared with those of the Mg alloy sheet. For the deformed specimens, interface debonding does not occur and the diffusion layer shows only a few cracks.

• International Journal of Automotive Technology
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• v.1 no.1
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• pp.35-41
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• 2000

The crashworthiness of vehicles with finite element methods depends on the geometry modeling and the material properties. The vehicle body structures are generally composed of various members such as frames, stamped panels and deep-drawn parts from sheet metals. In order to ensure the impact characteristics of auto-body structures, the dynamic behavior of sheet metals must be examined to provide the appropriate constitutive relation. In this paper, high strain-rate tensile tests have been carried out with a tension type split Hopkinson bar apparatus specially designed for sheet metals. Experimental results from both static and dynamic tests with the tension split Hopkinson bar apparatus are interpolated to construct the Johnson-Cook and a modified Johnson-Cook equation as the constitutive relation, that should be applied to simulation of the dynamic behavior of auto-body structures. Simulation of auto-body structures has been carried out with an elasto-plastic finite element method with explicit time integration. The stress integration scheme with the plastic predictor-elastic corrector method is adopted in order to accurately keep track of the stress-strain relation for the rate-dependent model accurately. The crashworthiness of the structure with quasi-static constitutive relation is compared to the one with the rate-dependent constitutive model. Numerical simulation has been carried out for frontal frames and a hood of an automobile. Deformed shapes and the Impact energy absorption of the structure are investigated with the variation of the strain rate.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 1995.10a
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• pp.69-75
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• 1995

The deformation behavior of stainless steel-clad aluminum sheet metals under uniaxial tension has been investigated. The differences in mechanical properties such as elastic modulus, flow stress and plastic strain ratio, of component layers of the composite sheet gave rise to warping of the tensile specimens. The warping has been analyzed by FEM and the total force and momentum equilibria. The analyzed radii of curvature of the warped specimens were smaller than the measured data possibly due to elastic recovery during unloading. The differences in mechanical properties may also give rise to transverse stresses in the component layers. The transverse stresses have been analyzed on the assumption of isostrain and by the FEM in which the warping has been taken into account. The transverse stresses calculated by the FEM were lower than those by the isostrain hypothesis due to stress relaxation by the warping and turned out to be negligible compared with the longitudinal stresses. Consequently, the flow stresses of the composite sheets follow the rule of mixtures.

• Laser Solutions
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• v.5 no.2
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• pp.23-29
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• 2002

As automotive manufacturers have taken a growing more interest in tailored sheet metals for improving the rigidity, weight reduction, crash durability, and cost saving application of the tailored sheet metals to automotive bodies has been resently increased greatly. In this study, we investigated the characteristics of fatigue crack initiation behavior of laser welded sheet use for vehicle body panel. We experimented three types of specimens which were machined of the same base metal : one is 1.4㎜ thick, another is 1.6㎜ thick, the others is laser welded of the 1.4mm thick specimen and 1.6㎜ thick specimen. The results indicated that laser welded metal (1.4＋1.6㎜) is the best one for fatigue strength and fatigue life.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.04a
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• pp.460-464
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• 1997

By the used of a similar numerical method as in the previous paper, the forming limit stain of coatedsheet metals is investigated in which the FEM is applied and J2G(J/sab 2/-Gotoh's corner theory) is utilized as the plasticity constitutive equation. Coated two-layer sheets and sheets bonded with dissimilar sheets on both surface planes are stetched in a plane-strain atate, with various work-hardening exponent n-values and thicknesses of each layer. Processes of shear-band formation in such composite sheets are clearly illustrated. It is concluded that, in the coated state, the higher limiting strain of one layer is reduced due to the lower limiting stain of the other layer and vice, and does not necessarily obey the rule of linear combination of the limiting stain of each layer weighted according thickness.